Aluminum alloy galvanic anodes



United States Patent US. Cl. 204-197 6 Claims ABSTRACT OF THE DISCLOSURE Aluminum base alloy galvanic anodes for the cathodic protection of metal structure having a lower anodic potential and a higher current efiiciency and substantially free of pitting or crevice formation due to selective corrosion during their useful life as consumable anodes to protect the metal structures containing 0.01-0.2% by weight of mercury, 0.01-% by weight of zinc and 0.01- 2% by weight of lead.

For the cathodic protection of metal structures, especially iron and steel structures, from corrosion in corrosive environments, in general, a galvanic or a consumable anode, which has less noble electrode potential than metals to be protected, is electrically connected to the metal structure to be protected. Accordingly, a galvanic anode is consumed and the current for the cathodic protection flows into the metal to be protected.

As aluminum base alloy galvanic anodes which are known at present, there are an alloy comprising 3-6% by weight of zinc; an alloy comprising a total of 01-10% by weight of tin and/ or indium, an alloy obtained by adding 110% by weight of zinc to said alloy; and an alloy prepared by adding zinc or magnesium to an aluminummercury alloy.

The aluminum-zinc alloy has an anode potential of 0.9 to 1.0 v. (vs. Standard Calomel Electrode) and an anodic current efficiency of about 50%.

The aluminum-indium alloy has an anode potential of 1.0 to --1.10 v. and an anodic current efficiency of not more than 80%.

Further, an aluminum alloy containing zinc, indium and cadmium, which is considered to be the best commercial aluminum alloy galvanic anode known at present, has an anode potential of about 1.1 v. and an anodic current efficiency of 80-85%.

On the other hand, the aluminum-mercury alloy shows an anode potential of about -1.3 v., but its anodic current efiiciency is less than 60%, and it is said that an alloy obtained by adding magnesium to said alloy can be improved in anodic current efliciency to about 90%.

However, a magnesium containing alloy has a danger of ignition of a fire due to impact with other metallic materials and hence is not desirable as an alloy for the cathodic protection of constructions for handling inflammable materials.

Further prior aluminum-mercury base alloys containing zinc or magnesium does not show uniform corrosion and is liable to show pitting or selective corrosion.

Moreover, said alloy shows unnecessary degradation of anode, when used for a long period of time, thereby shortening the life of anode.

Nice

Since the aluminum-mercury alloy is relatively favorable in respect of anodic potential as a galvanic anode, the present inventors made various investigations by adding a variety of elements to said alloy, in order to discover aluminum alloys acceptable as galvanic anodes. As the result, the inventors have found an aluminum alloy galvanic anode which is free from the above drawbacks.

One object of the present invention is to provide an aluminum alloy galvanic anode which has a lower electrode potential and a higher current efiiciency than prior commercial aluminum base alloys and shows no pitting or selective corrosion.

Other objects will be apparent from the following description.

In order to accomplish these objects the present inven tion provides an aluminum base alloy containing 0.01- 0.2% by weight of mercury, 0.01-10% by weight of zinc and 0.01-2% by weight of lead based on the weight of the alloy, which is an excellent aluminum alloy galvanic anode having an anode potential of about 1.11 v. and an anodic current efiiciency of -95%, showing an uniform corrosion and maintaining a lower electrode potential for along period of time.

In the present invention, the mercury content of the alloy is desirably 0.01-0.2% by weight based on the weight of the alloy.

In case the mercury content is more than 0.2%, the alloy is quite active and starts reaction with moisture in air immediately after preparation.

Consequently, the fluify aluminum hydroxide is formed on the surface and the reaction generates heat. On the other hand, in case the mercury content is less than 0.01% the effect of addition of mercury is not observed and the electrode potential of the alloy becomes more positive.

The content of Zinc is preferably 0.01-10% by weight based on the weight of the alloy. In case zinc content is more than 10% no difference is observed in effect, but the quantity of generated electricity is reduced.

In case the zinc content is less than 0.01%, the alloy shows a lower anodic current efficiency.

That is, it is considered that zinc has the effect of improving the current efliciency.

Preferable amount of lead to be added is 0.01-2% by weight based on the weight of the alloy. By the addition of lead, the dissolution of the alloy becomes uniform, without apparent pitting or selective corrosion.

Thus, the addition of lead greatly improves the corrosion resistance of the alloy.

In case the amount of lead added is less than 0.01%, no such effect is observed, while even when the amount is more than 2%, not only no difference is observed in efficiencies but the uniform structure of said alloy is not attained.

The aluminum alloy of the present invention may be heat treated.

Though the commercial aluminum is usually employed for the preparation of said aluminum base alloy, a more satisfactory result is attained by use of a high purity aluminum.

The following examples illustrate the present invention without limitation thereof.

EXAMPLE 1 An alloy prepared by adding to aluminum, 0.05% by weight of mercury, 0.1% by weight of zinc and 0.2%

by weight of lead was used as a test sample. This sample was cast in a metal mold into the form of a rod, was shaved so as to have a diameter of 20 mm. and a length of 100 mm., was degreased and was then weighed, and the exposed area of the sample was maintained constant. Subsequently, the test sample was immersed in artificial sea water having the composition shown below, and the iron plate was used as the cathode.

of lead was tested in the same manner and for the same period as in Example 1.

In this test, the anodic current efficiency of the alloy was 95.3% and the anode potential was about -1.12 v. The corroded surface of the alloy was greatly improved like in Example 1.

Table 1 shows the results of tests effected in the same manner as in Examples 1 and 2.

TABLE 1 168 hrs. after application of electric current Anode Current potential, efficiency Alloy composition v. percent Number:

3. Al plus 0.05% wt. Hg plus 3% wt. Zn plus 0.2% wt. Pb --1. 12 91. 2 4 Al plus 0.05% wt. Hg plus 0.1% wt. Zn plus 1.0% wt. lb 1.11 02. 9 5 Al plus 0.05% wt. Hg plus 3% Wt. Zn l. 05 01.1 G. Al plus 0.05% wt. Hg plus 3% wt. Zn plus 0.5% wt. M -l. 07 01. 8 7 Al plus 0.05% wt. Hg plus 0.1% wt. Z11 plus 6% wt. Mg -1. 02. 7

The anodic current density was maintained at 1 ma./cm. by an external battery.

Artificial sea water composition:

The test period was 168 hours. The amount of electricity passed was measured by use of the copper coulometer. After the test, the sample was washed with water, was immersed for 23 minutes in concentrated nitric acid to remove corrosion product from the surface and was weighed to calculate the corrosion loss:

The current efficiency of the galvanic anode was calculated from the theoretical amount of corrosion obtained from the total amount of electricity passed and from the corrosion loss, and was 94.9%.

The anode potential was about 1.11 v. throughout the test period.

The corrosion of the galvanic anode was quite uniform compared with the case of the conventional alloy.

EXAMPLE 2 An aluminum alloy containing 0.05% by weight of mercury, 0.1% by weight of zinc and 0.01% by weight the lead content is 0.2%.

4. An anode as set forth in claim 2 in which the mercury content is 0.05 the zinc content is 0.1%, and the lead content is 0.01%.

5. An anode as set forth in claim 2 in which the mercury content is 0.05%, the zinc content is 3.0%, and the lead content is 0.2%.

6. An anode as set forth in claim 2 in which the mercury content is 0.05%, the zinc content is 0.1%, and the lead content is 1.0%.

References Cited UNITED STATES PATENTS 4/1935 Brown 204-148 5/1967 Reding et al l46 RICHARD O. DEAN, Primary Examiner US. Cl. X.R. 

